1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) which uses the magneto resistive effect.
2. Description of the Related Art
In recent years, although many memories which store therein data based on the new principle have been proposed, one of such memories is a magnetic random access memory which uses the tunneling magneto resistive (which will be referred to as TMR hereinafter) effect.
As a proposal of the magnetic random access memory, there is known, for example, “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell” by Roy Scheuerlein at. al, ISSCC2000 Technical Digest p. 128.
The magnetic random access memory stores therein data of “1” and “0” by using TMR elements. The basic structure of the TMR element has a tunneling barrier sandwiched between two magnetic layers (ferromagnetic layers).
Data stored in the TMR element is judged by whether the magnetizing states of the two magnetic layers are parallel or anti-parallel. Here, “parallel” means that magnetizing directions of the two magnetic layers are the same, and “anti-parallel” means that magnetizing directions of the two magnetic layers are opposed.
Usually, an anti-ferromagnetic layer is provided as one (fixed layer) of the two magnetic layers. The anti-ferromagnetic layer is a member which fixes the direction of magnetization of the fixed layer. Actually, therefore, data (“1” or “0”) stored in the TMR element is determined by the direction of magnetization of the other one (free layer) of the two magnetic layers.
When the magnetizing states of the TMR element are parallel, the resistance of the tunneling barrier sandwiched between the two magnetic layers constituting the TMR element becomes lowest. For example, this state is determined as the state “1”. Further, when the magnetizing states of the TMR element are anti-parallel, the resistance of the tunneling barrier sandwiched between the two magnetic layers constituting the TMR element becomes highest. For example, this state is determined as the state “0”.
In regard to a cell array structure of the magnetic random access memory, various structures are currently examined in view of realization of the high memory capacity, stabilization of the write/read operation and others.
For example, nowadays, there is known a cell array structure in which one memory cell is constituted by one MOS transistor and one TMR element (or MTJ (Magnetic Tunnel Junction) element). Furthermore, in order to realize stabilization of the read operation, there is known a magnetic random access memory which has such a cell array structure and stores one-bit data by using two memory cell arrays.
However, the memory capacity can be hardly increased by these magnetic random access memory. That is because one MOS transistor corresponds to one TMR element in these cell array structures.